As compared with expanded polystyrene resin molded products, expanded polyethylene resin molded products are excellent in impact resistance, and restoring force such as deflection and repeated stress-strain. Due to these characteristics, expanded polyethylene resin molded products are widely used as a packing material of precise components or heavy products. In addition, since expanded polyethylene molded products are excellent in heat resistance and oil resistance, they are widely used as automobile components such as impact absorbers, bumpers and floor spacers.
Expandable polyethylene resin particles obtained by impregnating a polyethylene resin with a foaming agent are known. Polyethylene, however, tends to allow a foaming agent to pass therethrough. Therefore, several hours after the production, a foaming agent scatters, resulting in a significant decrease in foaming properties. Accordingly, it is required to subject such expandable polyethylene resin particles to a preliminary foaming shortly after the production to obtain expanded particles. For this reason, it is necessary to provide a preliminary foaming machine or a molding machine near a foaming agent impregnation equipment, which results in the restriction of the production base. Accordingly, transportation expenses of foamed particles or molded products are expensive, resulting in an economical disadvantage.
In addition, expanded olefin resin molded products are poor in compressive strength as compared with an expanded styrene resin molded product. Therefore, it is required that expanded olefin resin molded products have an expansion ratio lower than those of expanded polystyrene resin molded products. Accordingly, expanded olefin resin molded products are disadvantageous in costs as compared with expanded styrene resin molded products.
As a method for solving these problems, a method is proposed in which a cross-linking agent is added to polyethylene resin particles, and the resulting mixture is heated in a dispersion medium composed of water, low-boiling-point alcohols and ketones to allow the polyethylene resin particles to be cross-linked while the dispersion medium is impregnated into the cross-linked resin particles as a foaming agent (Patent Document 1).
In addition, many methods have been proposed in which a vinyl aromatic monomer in which a polymerization initiator and a cross-linking agent are dispersed is impregnated into polyethylene resin particles to conduct polymerization and cross linking, thereby obtaining modified polyethylene resin particles (Patent Documents 2 to 7).    Patent Document 1: JP-A-S50-139167    Patent Document 2: JP-A-S52-32990    Patent Document 3: JP-B-S45-32623    Patent Document 4: JP-A-H01-284536    Patent Document 5: JP-A-S48-101457    Patent Document 6: JP-A-S49-5473    Patent Document 7: JP-A-S49-97884
However, the method disclosed in Patent Document 1 has the disadvantage that foaming properties are significantly low, although a foaming agent can be reliably retained. That is, low-boiling-point alcohols or ketones do not function well as the foaming agent of polyethylene resin particles.
On the other hand, in the methods disclosed in Patent Documents 2 to 7, although retainability of a foaming agent and strength of an expanded molded product are improved by increasing the ratio of a vinyl aromatic monomer to polyethylene resin particles, it is required to increase significantly the ratio of a vinyl aromatic monomer to obtain a sufficient retainability of a foaming agent. As a result, impact resistance and restoring force such as deflection and repeated stress-strain, which are characteristics of polyethylene, cannot be obtained.
An object of the invention is to provide an expandable polyethylene resin particle which is improved in a long-term foam moldability, and an expanded polyethylene resin molded product which can exhibit high strength while keeping impact resistance and restoring force such as deflection and repeated stress-strain, which are characteristics of an expanded olefin resin molded product.